1. Field of the Invention
The present invention relates to an antenna for wireless communications. More particularly, the present invention relates to a small and omni-directional biconical antenna for use in for mobile communications.
2. Description of the Related Art
Wireless communications using an impulse (impulse communications) use a very wide frequency band, as compared to conventional narrow band wireless communications. In addition, impulse communications are known as a communication method enabling high-speed data transmission at a very low electric power. Previously, impulse communications have been applied to the field of radar. In an effort to improve performance of radar, studies have been performed to obtain a wide band operation and a high gain in addition to an antenna radiation pattern.
With the rapid development of mobile communications technologies, however, studies regarding the merits of applying impulse communications to the field of mobile communications have been actively undertaken. Even if impulse communications have superior technical merits, impulse communications cannot be applied to mobile communications when impulse communications inconvenience users who use the actual equipment or the equipment is difficult to carry. Thus, a first priority, prior to the application of impulse communications to mobile communications, is to provide a compact antenna for transceiving an impulse, i.e., an impulse antenna.
With the developments of relevant studies, a variety of types of impulse antennas have been suggested. FIGS. 1 through 3 illustrate examples of conventional impulse antennas.
FIG. 1 illustrates a perspective view of a conventional biconical antenna having a wide band feature.
An impulse antenna 10 includes an upper conductive body 11 and a lower conductive body 12 having a common power feed point 13. The upper and lower conductive bodies 11 and 12 are conical. The size of the impulse antenna 10 is designed by considering the minimum wavelength of an impulse in use. The length of the impulse antenna 10, that is, the length between the power feed point 13 and an edge of the impulse antenna 10, is designed to be at least ¼ of the wavelength of the minimum frequency of the impulse. However, since air is present between the upper conductive body 11 and the lower conductive body 12, the length R1 of the upper conductive body 11 and the length R2 of the lower conductive body 12 is more than ¼ of the wavelength in air of the minimum frequency included in the power feed signal.
In FIG. 1 and throughout the figures, angle θ1 denotes an angle between a Z-axis (not shown) passing through the center of the impulse antenna 10 and the upper conductive body 11. Angle θ2 denotes an angle between the Z-axis and the lower conductive body 12.
FIG. 2 illustrates a sectional view of an impulse antenna using a transverse electromagnetic (TEM) horn antenna. The impulse antenna shown in FIG. 2 is used for feeding of a pulse radar that is specially designed for a large output of power. A boundary surface 30 is angled with respect to a horizontal axis (not shown) so that a wave incident on the boundary surface 30 can be input at a Brewster angle. For a plane electromagnetic wavefront incident on a plane boundary between two dielectric substances having different refractive indices, a Brewster angle is the angle of incidence at which there is total transmittance from a first dielectric substance to a second dielectric substance.
However, a TEM wave input to the boundary surface 30 from the left side of the drawing is close to a spherical wave, not a plane wave. Accordingly, over the entire boundary surface 30, the incident angle of the TEM wave on the boundary surface 30 does not match the Brewster angle. As a result, a perfect impedance match is not made at the boundary surface 30. Impedance reflection due to the impedance mismatch at the boundary surface 30 increases as a height H2 of the TEM horn antenna increases.
In FIG. 2, reference numeral 1 denotes an electromagnetic wave generator; reference numeral 2 denotes a spark gap; reference numeral 3 denotes a pulser; reference numerals 6 and 14 denote grounded plates; reference numeral 8 denotes a parallel upper plate; reference numerals 10 and 17 denote dielectric materials; reference numerals 12 and 18 denote TEM horns; and reference numeral 16 denotes an upper plate. Further, distances H1 through H3 indicate gaps between the grounded plate 14 and the upper plate 16 in the TEM horn 18, the upper plate 16 and the grounded plate 14 in the TEM horn 12, and the upper plate 8 and the grounded plate 6 in the electromagnetic wave generator 1, respectively. Angles ψ1 and ψ2 indicate angles between the boundary surface 30 and a portion extending from the TEM horn 12 of the grounded plate 14 to the TEM horn 18, and the boundary surface 30 and an extended portion of the upper plate 16, respectively.
FIG. 3 illustrates a sectional view of a conventional biconical antenna 20 in which a dielectric material 33 having a dielectric constant ∈1 is used between an upper conductive body 26 and a lower conductive body 24. The dielectric material 33 prevents rain from flowing in along a power feed line when the biconical antenna 20 is used outdoors and simultaneously supports the upper and lower conductive bodies 26 and 24.
In FIG. 3, reference numerals 21, 23, and 24 denote a coaxial feed, a lower support structure, and a lower cone, respectively. Distances R1 and R2 indicate lengths of the upper conductive body 26 and the lower conductive body 24, respectively. Distances L′, L″, and L0 indicate lengths of an upper portion, a lower portion, and a middle portion of the dielectric material 33, respectively. Angle θ0 denotes an angle between the Z-axis and the middle portion of the dielectric material 33.
In a case of a conventional impulse antenna, a length of the antenna can be designed to be at least ¼ of the wavelength of the minimum frequency of a usable impulse. However, considering that the wavelength is in air, the size of the conventional impulse antenna is much greater than that of an antenna for a mobile communication terminal. In addition, in the conventional impulse antenna, since the TEM wave cannot be incident on the boundary surface at the Brewster angle, impedance mismatch is generated on the boundary surface, thereby generating an impulse reflection on the boundary surface, thus sharply deteriorating the quality of communication.